Backflow preventer apparatus are well known in the art and are used in water distribution systems to prevent contamination of the potable water portion by preventing backflow of liquid from the nonpotable portion beyond a certain point in the system. These backflow preventers are usually provided with one check valve or two check valves in series in a flow passage, the one or two check valves permitting flow of water from a supply pipe to a service pipe but preventing the backflow of liquid from the service pipe to the supply pipe during certain conditions such as "back siphonage" in the supply pipe or "back pressure" in the service pipe. The check valves usually include an annular valve seat provided in the body member of the same and a spring urged to closed position movable valve element for coacting with the seat.
In the more recent backflow preventer apparatus, the use of a preloaded valve module for the movable valve element has been used primarily where the movable valve element is a poppet-type valve element, i.e., a valve disc having a valve stem, a spring retainer and a coil spring held restrained at one end by a spring retainer and at the other end by the disc.
The springs used in such preloaded valve modules are quite highly loaded, for example, in an 8 inch valve, the loading of the spring would be approximately 370 pounds or more. The preloaded valve modules are installed in the body member at the manufacturing facility and thus, the entire check valve unit which forms a portion of the backflow preventer is installed at the site of use without removal of the valve module. However, oftentimes in the field a service person will try to service the valve module should the check valve leak. Because of the extreme load on the spring, this could be a very dangerous situation if the valve module can be indiscriminately disassembled. Another danger, which could occur, would be a situation where the valve stem failed and the cover plate of the body member was removed. The spring would have nothing to restrain it and once the bolts for the cover plate are free of the valve body member, there could be injury to the service personnel by the spring expanding and causing the cover plate to eject with considerable force.